CHT and gearing down to help cooling, myth OR fact, Get out your facts and scientific equations Options

[nditiz1]

It has been burned into my brain when I read it the first time that when going up a hill drop a gear (5th to 4th) to increase cooling thereby lowering the temps of the engine. Raby, Mark Henry, I'm sure others have stated the same thing.

I must admit my sample size is small (3 cars) AND 2 of which already had pre-existing issues with the engine health, but in all 3 scenarios when cruising on the highway going 75/80 dropping a gear down only momentarily lowers the temps only to shoot them back up possibly even higher as I am trying to maintain speed at a lower gear which requires more pedal. In each of the 3 cases I have been using a CHT over cyl 3. Two of which were a Dakota Digital and one was an aircraft spruce analog gauge. The one engine that was not plagued with less ponies was a rebuilt 2056 (by me). Maybe I built the engine wrong, I don't know. I guess I would like to hear what others have experienced. I am also not doubting the physics. More RPM = More fan turning = More cooling. Lowering gearing means the engine does not have to work as hard to pull up the hill. That last statement should have a caveat though. There must be a certain point of force vs gear ratio vs drag vs speed that would determine that staying in 5th would ultimately be more beneficial than dropping to 4th as far as work the engine is doing (probably a question for Phil - @superhawk996 )

So am I wrong in my limited testing and it does in fact shed heat driving on the highway 75-80 mph with ambient 85 - 90 or was it just a myth started because the physics seem to point that direction.

[emerygt350]

I went digging the other day for a 'real' value on head damage and I could find nothing but anecdotes (from good people, like Raby for example). 450 seemed to be the do not exceed value (although not a guaranteed death) 500 being the 'you have done something terrible', and 400-425 being the 'better watch that Mr" area. Pretty much don't cruise at anything 400 or above. I wonder about the absoluteness of CHT though. AFR and advance can all cause issues but I wonder if that is reflected 100% in CHT. For example, if you are not lean, and you are at 400 is everything fine and if you are lean and you are at 400 is everything not ok. Is it ok to be at 420 if you are rich? Or would you not have reached 420 if you were rich? Get what I mean? Should you feel confident that you are OK if the CHT says you are not in the danger zone or does that zone change depending on the tune. Seems to me that valve seats and stems are just gonna care about the temp.

And I really have to think about all those years of VW buses and 914s without CHTs, flogging the crap out of them, and still people feeling the engines were good and pretty well bullet proof.